This invention relates to a filter, a process for forming this filter, and a solid state imager provided with this filter. This invention is especially directed to color filters.
To obtain color image recording using solid state imagers such as charge coupled devices, optical filters in a multicolor stripe or mosaic form are employed; in many cases, these filters are formed directly upon the photosensitive surface of the solid state imager. Such filters are normally provided with elements having two or three differing colors. (The term "having color" is used herein to mean "transmitting electromagnetic radiation of a particular wavelength", and does not necessarily refer to visible radiation. Thus, a three color filter of this invention may have elements which transmit three differing wavelengths of ultraviolet radiation, all of which are invisible to the human eye.) For example, a two color filter may have yellow and cyan elements which overlap in part, the overlap area providing, in effect, a green element. A three color filter will typically have red, green and blue, or cyan, yellow and magenta elements.
A number of processes are described in the art for preparing such filters. For example, U.S. Pat. No. 4,239,842, issued Dec. 16, 1980, describes a process for producing a color filter array by depositing successively on a semi-conductive layer, such as a charge coupled device, a sub-coat, a polymeric mordant, and a photoresist. The photoresist layer is exposed and developed to form a mask, and dye is then heat-transferred through the apertures in the photoresist into the polymeric mordant. Finally, the photoresist is stripped.
U.S. Pat. No. 4,565,756, issued Jan. 21, 1986, describes a color filter formed by laying on a substrate a transparent layer, forming by photolithography a pattern of filter elements separated by separation regions (grooves or dye-impermeable regions) in the transparent layer, laying a barrier layer over the transparent layer, forming by photolithography a pattern of apertures in the barrier layer, this pattern of apertures corresponding to the location of a first system of filter elements, dyeing the first system of filter elements through these apertures, and finally removing the barrier layer. The formation of the barrier layer and the subsequent steps of the process are then repeated for other colors.
A variety of techniques have also been developed for producing the fine lines and other image elements needed in the production of integrated electrical circuits and in lithography. For example, U.S. Pat. No. 3,873,361, issued Mar. 25, 1975, describes a process for producing thin films for integrated circuits by depositing (a) a photoresist (polymeric) layer which is baked to render it non-photosensitive; (b) a metallic layer; (c) a second photoresist layer, then exposing and developing the second photoresist layer to form a mask, etching the exposed metallic layer through this mask, using the metallic mask so produced to remove exposed polymeric layer, preferably by sputter etching, depositing a metallic film in areas where the bottom polymeric layer has been removed, and finally removing, by conventional lift-off solvent methods, the remaining parts of the metallic layer and the bottom polymeric layer.
U.S. Pat. No. 4,428,796, issued Jan. 31, 1984, describes the production of integrated circuits by coating a silicon substrate with layers of polyimide material, silicon dioxide and positive photoresist. The photoresist is exposed and developed. Using plasma or reactive ion etching, the silicon dioxide is etched away where the photoresist has been removed. A different plasma is then used to etch the polyimide where the silicon dioxide has been removed. A desired (typically electrically conductive) material is placed on the substrate so that discrete portions of the desired material are formed in the holes in the polyimide layer and on top of the silicon dioxide layer (the photoresist layer disappears during the second etching). The remaining parts of the polyimide layer, together with the overlying silicon dioxide and desired material layers, are then stripped away by heating, treatment with solvent, and ultrasonification.
U.S. Pat. No. 4,891,303, issued Jan. 2, 1990 on application Ser. No. 07/199,087, filed May 26, 1988, describes a method for patterning an integrated circuit workpiece including forming a first layer of organic material on the workpiece surface to a depth sufficient to allow a substantially planar outer surface thereof. A second, polysilane-based resist layer is spin-deposited on the first layer. A third resolution layer is deposited on the second layer. The resolution layer is selectively exposed and developed using standard techniques. The pattern in the resolution layer is transferred to the polysilane layer by either using exposure to deep ultraviolet or by a fluorine-base reactive ion etching (RIE) etch. This is followed by an oxygen-based RIE etch to transfer the pattern to the surface of the workpiece.
Similar three-layer photoresist systems are described in O'Toole et al., Multilevel resist for photolithography utilizing an absorbing dye; simulation and experiment, The International Society for Optical Engineering (SPIE), Volume 275, Semiconductor Microlithography VI (1981) (which discusses the use of such systems in the production of integrated circuits); in Paraszczak et al., Chemical and physical aspects of multilayer resist processing, SPIE Volume 920, Advances in Resist Technology and Processing V (1988); and in Underhill et al., Silicon oxynitride as a barrier layer in a 3-layer photoresist system, SPIE Volume 539, Advances in Resist Technology and Processing II (1985).
U.S. Pat. No. 4,808,501, issued Feb. 28, 1989 and assigned to the same assignee as the present application, describes a process for forming a color filter on a support, such as a charge coupled device, by (a) forming a layer on a support with a composition comprising a positive photoresist and a dye, the dye being soluble in the solvent of the photoresist; (b) exposing predetermined portions of the layer to radiation adapted to increase the solubility of the coating in the exposed areas; (c) developing the exposed areas to form a pattern of filter elements; and (d) repeating these steps with a different color dye in the composition; wherein the dye constitutes in excess of 10% by weight, dry basis of the composition, is substantially non-absorptive in the exposure wavelength of the composition, and provides predetermined absorptive characteristics for the specified filter element and the dye possesses substantially the same polarity as the composition.
The process described in U.S. Pat. No. 4,808,501 gives excellent results. However, this process does require that the dye be substantially non-absorptive in the exposure wavelength of the composition, and this creates difficulties in forming a yellow filter element with some commercial photoresists. Many commercial Novolak photoresists are designed for exposure using the g-line of a mercury lamp at 436 nm., and yellow dyes tend to absorb this line strongly. Thus, these yellow dyes, when used at the very high concentrations required in the process of U.S. Pat. No. 4,808,501, require very long exposure times, since the high concentration of yellow dye in the photoresist absorbs much of the 436 nm. light used to expose the photoresist.
The present invention provides a process for forming a filter on a substrate which does not require the use of a dye suitable for incorporation in a photoresist composition, and which can thus be used with dyes which absorb strongly at the exposure wavelength of a photoresist, or which chemically disrupt a Novolak positive process. The present process can be used alone to form multicolored filters, or filter elements of one color can be formed by the present process and filter elements of one or more other colors can thereafter be formed using the process of U.S. Pat. No. 4,808,501. The present process is especially useful for forming a filter element directly on a solid state imager, since it does not require any conditions detrimental to a solid state imager. The present invention is also capable of providing filters of high quality in which the filter elements are accurately aligned with the photosensitive elements of the solid state imager, and in which there is little or no undesired overlap between filter elements of differing colors.